The economic burden on a child born in 2015 will be nearly twice that of a child born in 1985, according to a University of Southern California study. / RICK DANZL/Associated Press

By Haya El Nasser

USA TODAY

The drop in U.S. births to their lowest level since 1920 is sounding alarms about the nation's ability to support its fast-growing elderly population.

As public concern mounts, a growing number of books, reports and columns are laying out challenges the United States will face because of this demographic upheaval: Fewer babies are being born while the wave of 78 million older Baby Boomers have only begun to retire (the oldest turn 67 this year).

“What to Expect When No One's Expecting: America's Coming Demographic Disaster,” a book by Jonathan Last, went on sale this month. A University of Southern California study out last month reported an unprecedented decline in California's child population that "will pose significant challenges for the state's future prosperity."

The recent decline, fueled largely by a deep recession and slower immigration, has pushed the U.S. fertility rate below the 2.1 "replacement level'' — the number of children women are expected to have in their lifetime if current rates continue and the number needed to keep the population stable.

The slowdown is worrisome to many because of the growing gap between working-age populations that fund social programs and the elderly who rely on them.

The imbalance between children and retirees is growing. The economic burden on a child born in 2015 will be nearly twice that of a child born in 1985, according to the USC study.

"These are two trends going in the opposite direction," says demographer Dowell Myers, director of the Population Dynamics Research Group at the University of Southern California. "We will be increasingly dependent economically and socially on a smaller number of children."

In California, where about half of babies are born to immigrant mothers, the immigration slowdown is having a big effect, and it's not clear whether proposed changes to immigration laws will change the pattern. White House and congressional proposals would give illegal immigrants a way to become citizens but that would affect those already here.

"It's totally up in question to what extent immigration will return," says Last, also a senior writer for the Weekly Standard. "All the energy goes out of society when fertility rates get low."